In recent years, techniques of industrial robots have progressed rapidly, and consequently, a positioner, one example of robot supporting peripherals, has drawn attention from the market.
A conventional positioner is described with reference to FIG. 4.
FIG. 4 illustrates a construction of the conventional positioner exclusively used for welding. In FIG. 4, rotary table 21 made from conductive material receives jig 32 thereon and rotates jig 32. A work-piece to be processed is positioned with jig 32. Conductive pipe 22 is press-fitted as a rotary shaft into the center of rotary table 21. Conductive collector ring 23 is fixed to a section near to the other end of pipe 22, and a lower face of a collector ring is finished flat and contacts carbon brush 24. Carbon brush 24 is shaped as a ring, and is urged by spring 25 upwardly so that carbon brush 24 keeps contacting with collector ring 23 with a given contact-pressure, and welding current thus runs across carbon brush 24 and collector ring 23. Carbon brush 24 is placed concentrically with rotary shaft 22 in such a manner that it does not directly contact shaft 22. Pins 26 fix carbon brush 24 at its lower face in order to avoid being rotated by frictional force created at the contact face with collector ring 23. Carbon brush 24 is coupled to a welding-power-supply disposed outside with cable 34. Motor 27 drives rotary table 21 and has first pulley 28 on its spin shaft. Rotary shaft 22 has second pulley 29. First and second pulleys 28 and 29 are coupled with timing belt 30 so that the spin of motor 27 is transferred to second pulley 29 via first pulley 28 and timing belt 30, and shaft 22 is thus rotated. Jig 32 mounted on rotary table 21 is usually coupled with hoses, e.g. through which compressed air travels for clamping the work-piece, as well as coupled with cables from a sensor detecting the work-piece. These hoses and cables 33 run through hollow-shaft 22 before being coupled to the work-piece. Outer housing 31 has windows through which a user can monitor an inside operation.
The positioner constructed as discussed above, has the following advantages over a positioner having a carbon brush disposed on an outer rim of a collector that is in contact with the collector ring: the body is smaller in a radial direction, a flat contact face between the collector ring and carbon brush can be easily processed, and its flatness can be easily maintained. The positioner having this structure has been thus widely used.
However, this conventional positioner still has problems, i.e. carbon brush 24 wears out because of abrasion at the contact face with collector ring 23 due to the rotation of rotary table 21. The remaining abrasive powder on the contact face sometimes makes the contact between collector ring 3 and carbon brush 24 unstable. This unstable contact increases a voltage drop on the contact face, which makes welding current unstable. As a result, defective welding occurs. In an extreme case, the contact face would spark, which can damage significantly both the contact faces of the collector ring 23 and carbon brush 24. Therefore, the abrasive powder on carbon brush 24 must be removed, and the damaged contact faces must be repaired, so that additional maintenance cost is required.
Further, the maintenance work, such as replacing the worn-out carbon brush 24, and repairing the contact faces of collector ring 23 and carbon brush 24, requires cumbersome work such as dismounting the rotary shaft 22 in order to remove these elements.